The prior art has concerned itself, for many years, with the problem of reducing or eliminating the loss of water in galvanic cells using aqueous electrolyte and avoiding build up of excessive gas pressure in sealed cells. Oxygen gas is evolved during overcharge or cell reversal.
Several galvanic couples are known with a capability of oxygen recombination cycles; however, the recombination rates are not always as fast as might be desired. PbO2-Pb and Ni-Cd, for example, show high rates of oxygen recombination, while batteries employing Zn anodes (Ni-Zn), MnO2-Zn, HgO-Zn) exhibit low rates. Reference to the theory of this lead-acid battery oxygen recombination for instance is made in "Batteries, Vol. 2, Lead-Acid Batteries and Electric Vehicles" pp. 69-61 by K. Kordesch; and to the Ni-Cd couples in "Alkaline Storage Batteries", by A. Salkind. The reduction to practice, though, has not been easy and a variety of difficulties has been reported: such as sealing problems, negative "fade" (e.g. passivation of the negative cadmium electrode with time), gradual decrease in effectiveness of the negative active material reserve which is provided in some designs in order to minimize--if not entirely suppress--hydrogen evolution during charge and overcharge, the general problem of coping with the recombination of nonstoichiometrically evolving oxygen and hydrogen, etc.
Three approaches are often used in efforts to solve these problems. These approaches are:
(1) Operation of the "oxygen cycle". Hydrogen evolution is suppressed and the evolved oxygen (during charge and overcharge) is recombined at the always partially discharged negative electrode [U.S. Pat. No. 3,258,360 (1966)]. PA0 (2) Catalytic recombination of hydrogen and oxygen inside or outside the battery; in the latter case, provisions are made for the return of the product water to the electrolyte chamber [U.S. Pat. No. 3,630,778 (1971), U.S. Pat. No. 3,598,653 (1971), U.S. Pat. No. 3,622,398 (1971), U.S. Pat. No. 3,701,691 (1972)]. PA0 (3) Use of an auxiliary (third) electrode as overcharge recombination reactors, as described in "Electrochem. Technol.," 4, 383 (1966) by P. Ruetschi and J. B. Ockerman.
It has now been unexpectedly discovered that the addition of a disc of carbon bonded with polytetrafluoroethylene (PTFE) as an electrochemical, transfer electrode for the anode mass, has the effect to enhance oxygen recombination at the anode.
According to the present invention, there is provided a rechargeable electrochemical cell having a metal oxide cathode, a zinc anode, and an aqueous alkaline electrolyte contacting the anode and the cathode, in which cell oxygen may evolve on charge, overcharge, or any reversal of cell polarity. The cell includes an auxiliary, electrochemical, transfer electrode for the anode. The auxiliary electrode is physically separated from the anode but is in electronic and ionic contact with it, and is at least partially wetted by the electrolyte. The auxiliary electrode comprises porous carbon bonded with polytetrafluoroethylene.
The metal oxide of the cathode may be manganese dioxide, which may be mixed with graphite in an amount of from 5% to 20% by weight, or may be mixed with nickel oxide in an amount of from 10% to 20% by weight. Alternatively the metal oxide may be nickel oxide, silver oxide or iron oxide and may be mixed with graphite in an amount of from 5% to 20% by weight.
The present invention may provide economic and effective means of reabsorbing oxygen gas in galvanic cells.
Embodiments of the invention will now be described by way of illustration with reference to the drawings in conjunction with the Example, describing an electrode of the invention, and its operating characteristics.